Resonators are widely used in situations where electromagnetic signals or fields are being generated or detected. A body made of a conductive material with a dielectric substrate will resonate at a particular frequency, with that resonant frequency corresponding to a particular wavelength that is related to the dimensions of the body and the material properties of the body. Where the body is effectively one-dimensional, this relationship is relatively straightforward. However, where the body is two- or three-dimensional, or where there is coupling between two or more such bodies, the relationship becomes more complex.
In one well-known form of resonator, a generally square, thin (that is, effectively two-dimensional) layer, or patch, of conductive material is provided. An effectively two-dimensional body of this type has two resonant modes, relating to oscillations along the width and the length of the patch respectively. In each case, the resonant frequency of the mode corresponds to a wavelength which is approximately double the respective dimension of the patch.
By removing a part of the conductive material, it is possible to establish a degree of coupling between these two resonant modes. For example, a generally square area of conductive material can be removed from a corner of the patch. The degree of coupling affects the overall frequency response of the patch resonator. If the degree of coupling is at a critical level, the frequency response includes a resonance at a particular frequency. If the degree of coupling is below this critical level, the resonator becomes less efficient. If the degree of coupling is above the critical level, the resonant peak is effectively split into two peaks and spread over a wider range of frequencies, that is, the resonator has a larger bandwidth.